In the first study, DNA polymerase iota (Pol i) has harbored significant interest as a component of somatic hypermutation in antibody genes because of its low fidelity. Although there was no effect on mutation in mice deficient for the enzyme (strain 129, Poli 129/129), we hypothesized that a protein that is expressed but catalytically inactive might unmask a role for Pol i. A mutant Pol i was designed with alanine substitutions at residues D126 and E127 (Pol im), which abolished its catalytic activity in biochemical assays. A knock-in mouse (Poli m/m) was constructed, and germinal center B cells were examined for mutations. Compared to Poli +/+ mice, the mutation frequency and spectra were not different. However, because we had previously shown that Pol zeta generates tandem mutations, we analyzed their frequency. Surprisingly, Poli m/m mice had fewer contiguous mutations compared to Poli +/+ and Poli 129/129 mice, suggesting that Pol im blocks access of Pol z to mismatched termini. We propose that wild type Pol i binds to a primer-terminus and needs to be displaced, so that Pol z can synthesize. In the second study, mammalian ATAD5 and its yeast homolog ELG1 are responsible for unloading PCNA from newly synthesized DNA. Prior work in HeLa and yeast cells showed that a decrease in ATAD5 protein levels resulted in accumulation of chromatin-bound PCNA, slowed cell division and increased genomic instability. In this study, B cells from heterozygous (Atad5+/m) mice were used to examine the effects of decreased cell proliferation on antibody diversity. ATAD5 haploinsufficiency did not change the frequency or spectrum of somatic hypermutation in antibody genes, indicating that DNA repair and error-prone DNA polymerase eta usage were unaffected. However, immunized Atad5+/m mice had decreased serum IgG1 antibodies, demonstrating a functional effect on class switch recombination. The mechanism of this altered immune response was then examined following ex vivo stimulation of splenic B cells, where Atad5+/m cells accumulated in S phase of the cell cycle and had reduced proliferation compared to wild type cells. These haploinsufficient cells underwent a significant decline in activation-induced deaminase expression, resulting in decreased switch region DNA double-strand breaks and inter-chromosomal translocations in the Igh locus. Class switch recombination to several isotypes was also reduced in Atad5+/m cells, although the types of end-joining pathways were not affected. These results describe a defect in DNA replication that affects Igh recombination via reduced cell division.